I have SQL Server 2008 installed on my machine and also Reporting Services Configuration Manager. When I connect to SQL Server, I found two databases already there.
I know ReportServer is to store reports, data sources, snapshots, subscriptions, etc. But what is ReportServerTempDB for? Why is it created? Is that necessary (for our use)?
Read the documentaion on report server database
The databases are created together and bound by name. By default, the database names are reportserver and reportservertempdb, respectively.
Report Server Temporary Database
Each report server database uses a related temporary database to store session and execution data, cached reports, and work tables that are generated by the report server. Reporting Services does not re-create the temporary database if it is missing, nor does it repair missing or modified tables. Although the temporary database does not contain persistent data, you should back up a copy of the database anyway so that you can avoid having to re-create it as part of a failure recovery operation. If you back up the temporary database and subsequently restore it, you should delete the contents. Generally, it is safe to delete the contents of the temporary database at any time. However, you must restart the Report Server Windows service after you delete the contents. If you delete the temporary database, you can create a new database, and then run the Catalogtempdb.sql script to add the table structure. The temporary database must have the same root name as the primary report server database.
